Assembled camshafts are known which comprise an inner shaft and an outer tube that are rotatable relative to one another. A first set of cams is secured for rotation with the outer tube while a second set of cams is rotatably mounted on the outer tube and connected for rotation with the inner shaft by way of pins that pass through slots in the outer tube that extend circumferentially. Such a camshaft, which allows the relative phase of adjacent cams rotatable about a common axis to be changed, is known (for example from EP-A-1 362 986) and is commonly and herein referred to as a single cam phaser (abbreviated to SCP) camshaft.
There are also known hydraulically operated vane-type cam phasers that are intended to drive an SCP camshaft, an example of such a phaser being disclosed in U.S. Pat. No. 6,725,817. Such phasers will herein be referred to as twin phasers, because they have two output members, one for driving the inner shaft of the SCP camshaft and the other for driving its outer tube. The phase of at least one, or more preferably both, of the output members are adjustable hydraulically relative to the engine crankshaft, such as by controlling the flow of oil under pressure to arcuate working chambers arranged on opposite sides of radial vanes connected to a respective one of the output members.
The present invention is concerned with the manner in which a twin phaser is fitted to an SCP camshaft.
Reference will now be made to FIG. 1, which is an axial section showing a twin phaser 10 mounted on an SCP camshaft 30 in a known manner, to explain some of the problems encountered in the prior art.
In FIG. 1, the twin phaser 10 has a stator 12 fitted with a sprocket 20 to be driven by the engine crankshaft. Front and rear end plates 14 and 16 are connected to radial vanes (not shown) that are movable in arcuate working chambers in the stator 12 and serve as output members.
The internal construction of the phaser 10 is not shown in detail in FIG. 1, the only part showing in the section of the drawing being a spring loaded pin 18 for locking the front plate 14 to the stator 12 under certain conditions.
The rear end plate 16 is coupled by means of dowel pins 22 to a bearing 24 that is fast in rotation with the outer tube 26 of the SCP camshaft 30. The outer tube 26 is fast in rotation with some of the cam sleeves, such as the cam sleeve 28. Other cam sleeves, such as the sleeve formed with two cam profiles 32 and 34 are coupled by driving pins 36 for rotation with the inner shaft 40 of the SCP camshaft 30. A nose portion 50, which is integral with or permanently secured to the inner shaft 40, passes through the stator 12 and receives a nut 44 that clamps onto the front end plate 14 of the twin phaser 10, whereby the inner shaft 40 rotates with the front end plate 14 while the outer tube 16 rotates with the bearing 24 and the rear end plate 16. The nose portion 50 is also formed with oil galleries 42 terminating in grooves which supply oil to the working chambers of the twin phaser 10.
Whilst the above provides a functional design solution, it presents certain problems which are addressed by the present invention and which will now be explained.
A first problem encountered in the prior art is additional friction between the inner shaft 40 and outer tube 26 of the SCP camshaft 30. The reason for this is that all the chain/belt loads from the sprocket 20 are transferred onto the cam nose 50 and then onto the bearing surface, designated 38 in FIG. 1, between the inner shaft 40 and the outer tube 26. This potentially affects the performance of the valve system by introducing undesirable friction between these two components of the camshaft 30.
A second problem in the prior art is that the inner shaft 40 is subjected to both bending forces and torque and needs to be supported inside the outer tube 26. This makes the SCP camshaft design very sensitive to manufacturing tolerances because the inner shaft 40 is located by both the bearings in the outer tube 26 and the connecting pins 36. The improved SCP camshaft design described in GB Pat. Appln. No. 0522328.7 requires the inner shaft 40 to be subjected to torque only.
Further problems with the prior art result from the fact that it is difficult to assemble the phaser 10 onto the camshaft 30. The assembly of twin phasers onto SCP camshafts is inherently more complex than the assembly of a standard sprocket to a solid camshaft. It is usually not possible to install the camshaft and phaser as one complete unit as a camshaft thrust control plate between these two parts. The fixings for the thrust plate are usually so arranged that the only method of assembly is first to install the camshaft in the engine, then to bolt the thrust plate in place and finally to assemble the phaser to the front of the camshaft with the chain and crank sprocket.
A third problem with the prior art is that the support bearing for the phaser assembly 10 is part of the cam nose 50 and therefore forms a part of the camshaft assembly. The bearing surfaces are thus exposed to dirt and debris during the assembly operation, and if these were to be contaminated, the phaser could be caused to malfunction. It is also impossible to properly test the twin phaser assembly 10 as a unit before it is fitted to the SCP camshaft assembly 30 because it only becomes united with its support bearing at the time of assembly.
A fourth problem with the prior art is that the two dowel pins 22 that are used to transmit torque from the rear plate 16 to the outer tube assembly 24, 26 are difficult to align and require tight manufacturing tolerances on both mating parts. Assembly of the twin phaser is therefore relatively complicated, requiring skilled manual procedures that could potentially slow down the production line.
A fifth problem with the prior art is that the phaser 10 is not clamped axially to the front bearing 24 of the SCP camshaft 30 because the driving connection is achieved with dowel pins 22. This means that the relative axial positions of the inner shaft 40 and outer tube 26 of the SCP camshaft need to be dictated by thrust control features on the SCP camshaft and cannot be controlled by the phaser.